1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor used for electrophotographic image formation and an image forming apparatus provided with the same.
2. Description of the Related Art
In electrophotographic image forming apparatus (hereinafter also referred to as an electrophotographic apparatus) used, for example, as a copying machine, a printer, or a facsimile apparatus, images are formed by way of the following electrophotographic process. At first, a photosensitive layer of an electrophotographic photoreceptor (hereinafter also referred to simply as a photoreceptor) provided in the apparatus is charged uniformly to a predetermined potential by a charger, and exposed to a light such as a laser light irradiated from exposure means in accordance with image information, to form electrostatic latent images. A developer is supplied from development means to the formed electrostatic latent images and colored fine particles referred to as toners which are a component of the developer are deposited on the surface of the photoreceptor to develop the electrostatic latent images and visualized as toner images. The formed toner images are transferred by transfer means from the surface of the photoreceptor to a transfer material, for example, recording paper and fixed by fixing means.
In the transfer operation by the transfer means, not all the toner on the surface of the photoreceptor are transferred and moved to the recording paper, but a portion thereof is remained on the surface of the photoreceptor. Further, a paper powder of the recording paper in contact with the photoreceptor during transfer sometimes remains being deposited as it is on the surface of the photoreceptor. Since obstacles such as residual toner and the deposited paper powder on the surface of the photoreceptor give undesired effects on the quality of the images to be formed, they are removed by a cleaning device. A cleanerless technique has been progressed in recent years and the residual toner is removed by a so-called development and cleaning system of recovering the same by a cleaning function added to the development means, with no independent cleaning means. After cleaning the surface of the photoreceptor as described above, the surface of the photosensitive layer is charge-eliminated by a charge eliminator to eliminate electrostatic latent images.
An electrophotographic photoreceptor used in such an electrophotographic process is constituted by laminating a photosensitive layer containing a photoconductive material on an conductive base body comprising a conductive material. As the electrophotographic photoreceptor, an electrophotographic photoreceptor using an inorganic photoconductive material (hereinafter referred to as an inorganic photoreceptor) has been used so far. Typical inorganic photoreceptor includes a selenium photoreceptor using a layer comprising an amorphous selenium (a-Se) or an amorphous selenium arsenide (a-AsSe) as a photosensitive layer, a zinc oxide or cadmium sulfide photoreceptor using zinc oxide (chemical formula: ZnO) or cadmium sulfide (chemical formula: CdS) together with a sensitizer such as a dye being dispersed in a resin as the photosensitive layer, and an amorphous silicon photoreceptor (hereinafter referred to as a-Si photoreceptor) using a layer comprising amorphous silicone (a-Si) as a photosensitive layer.
However, the inorganic photoreceptor has the following drawbacks. The selenium photoreceptor and the cadmium photoreceptor have drawbacks in view of the heat resistance and the store stability. Further, since selenium and cadmium have toxicity to human bodies and environments, the photoreceptors using them have to be recovered and discarded properly after use. Further, the zinc oxide photoreceptor has a drawback that it has low sensitivity and low durability and is scarcely used at present. Further, while the a-Si photoreceptor attracting attention as the inorganic photoreceptor with no public pollution has advantages such as high sensitive and high durability but since this is manufactured by using a plasma chemical vapor deposition method, it is difficult to uniformly deposit the film of the photosensitive layer and has a drawback tending to cause image defects. Further, the a-Si photoreceptor also has a drawback of low productivity and high manufacturing cost.
As described above, since the inorganic photoreceptor involves many drawbacks, development has progressed for the photoconductive material used for the electrophotographic photoreceptor, and organic photoconductive materials (that is, Organic Photoconductor: abbreviated as: OPC) have been now used frequently instead of the inorganic photoconductive materials used so far. While the electrophotographic photoreceptor using the organic photoconductive material (hereinafter referred to as organic photoreceptor) involves some problems in view of the sensitivity, durability and stability to environment, it has various advantages compared with the inorganic photoreceptor in view of the toxicity, the production cost and the degree of freedom for the material design. Further, the organic photoreceptor also has an advantage that the photosensitive layer can be formed by an easy and inexpensive method typically represented by a dip coating method. Since the organic photoreceptor has such various advantages, it has now gradually been predominant in the electrophotographic photoreceptors. Further, the sensitivity and the durability of the organic photoreceptor has been improved by the research and development in recent years and the organic photoreceptor has been used at present as the electrophotographic photoreceptor except for special cases.
organic photoreceptors are being developed by the development of function-separated electrophotographic photoreceptors of which charge-generating function and charge-transporting function thereof are separately attained by different substances. In addition to the above-mentioned advantages of organic photoreceptors, such function-separated photoreceptors have broad latitude in selecting the materials constituting photosensitive layer and have an advantage in that those having any desired characteristics are relatively readily produced.
The function separated type photoreceptor includes a lamination type and a single layer type. In the lamination type function separated photoreceptor, a lamination type photosensitive layer constituted by lamination of a charge-generating layer containing a charge-generating substance for charge generating function and a charge-transporting layer containing a charge-transporting substance for charge-transporting function is provided. The charge-generating layer and the charge-transporting layer are usually formed such that the charge-generating substance and the charge-transporting substance are formed respectively being dispersed in binder resins as the binding agent. Further, in the single layer type function-separated photoreceptor, a photosensitive layer of a single layer type formed by dispersing the charge-generating substance and the charge-transporting substance in a binder resin together is provided.
A variety of substances have heretofore been investigated for the charge-generating substances that may be used in the function-separated photoreceptors, including, for example, phthalocyanine pigments, squarylium dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes and pyrylium salt dyes, and various materials of good light fastness and good charge-generating ability have been proposed.
On the other hand, various compounds are known for the charge-transporting substances, including, for example, pyrazoline compounds (e.g., refer to Japanese Examined Patent Publication JP-B2 52-4188 (1977)), hydrazone compounds (e.g., refer to Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), and Japanese Unexamined Patent Publication JP-A 55-52063 (1980)), triphenylamine compounds (e.g., refer to Japanese Examined Patent Publication JP-B2 58-32372 (1983) and Japanese Unexamined Patent Publication JP-A 2-190862 (1990)) and stilbene compounds (e.g., refer to Japanese Unexamined Patent Publications JP-A 54-151955 (1979) and JP-A 58-198043 (1983)). Recently, pyrene derivatives, naphthalene derivatives and terphenyl derivatives that have a condensed polycyclic hydrocarbon structure as the center nucleus have been developed (e.g., refer to Japanese Unexamined Patent Publication JP-A 7-48324 (1995)).
The charge-transporting substances must satisfy the following requirements:    (1) they are stable to light and heat;    (2) they are stable to active substances such as ozone, nitrogen oxides (NOx) and nitric acid that may be generated in corona discharging on a photoreceptor;    (3) they have good charge-transporting ability;    (4) they are compatible with organic solvents and binder resins;    (5) they are easy to produce and are inexpensive. Though partly satisfying some of these, however, the charge-transporting substances disclosed in the above-mentioned patent publications could not satisfy all of these at high level.
Further, in recent years, higher sensitivity is required for the photoreceptor characteristic corresponding to the requirement for reduction of the size and increase of the operation speed to electrophotographic apparatus such as a digital copying machines and a printer, and a particularly high charge-transporting ability is demanded for the charge transpiration substance. Further, in a high speed electrophotographic process, since the time from the exposure to the development is short, it has been demanded for a photoreceptor of excellent light responsiveness. In a case where the light responsiveness of the photoreceptor is low, that is, the decaying speed for the surface potential after exposure is slow, the residual potential increases and the photoreceptor is used repetitively in a state where the surface potential is not decayed sufficiently, the surface charges at a portion to be eliminated are not eliminated sufficiently by exposure to bring about a drawback such as lowering of the image quality in the early stage. In the function separated type photoreceptor, since charges generated by the charge-generating substance due to light absorption are transported by the charge transpiration substance to the surface of the photosensitive layer thereby eliminating the surface potential of the photoreceptor at a portion irradiated with a light, the light responsiveness depends on the charge-transporting ability of the charge transpiration substance. Accordingly, a high charge-transporting ability is required for the charge-transporting substance also in view of attaining a photoreceptor having a sufficient light responsiveness.
For the charge-transporting substances that satisfy the requirement, proposed are enamine compounds having higher charge-transporting ability than that of the charge-transporting substances disclosed in the above-mentioned patent publications (e.g., refer to Japanese Unexamined Patent Publications JP-A 2-51162 (1990), JP-A 6-43674 (1994) and JP-A 10-69107 (1998)). Further, in another related art, incorporation of polysilane and an enamine compound having a specified structure to a photosensitive layer is proposed for improving hole-transporting ability of the photoreceptor (for example, refer to Japanese Unexamined Patent Publication JP-A No. 7-134430 (1995)).
Further, in the electrophotographic apparatus, since the operations of charging, exposure, development, transfer, cleaning and charge elimination to the photoreceptor are conducted repetitively, the photoreceptor is required to be excellent in the durability to electrical and mechanical external forces in addition to high sensitivity and excellent light responsiveness. Specifically, it has been demanded that abrasion and injury are not caused by friction with a cleaning material or the like to the surface layer of the photoreceptor and it is not degraded by deposition of active substance such as ozone and NOx generated upon electric discharge during the charged state.
In order to realize cost reduction and maintenance-free condition of the electrophotographic image forming apparatus, it is important that the electrophotographic photoreceptor has satisfactory durability and can be operated stably for a long period of time. One of factors that influences the durability and the long-term stability of the operation is surface cleanability, namely, ease of surface cleaning which is related with the surface condition of the electrophotographic photoreceptor.
The cleaning of the electrophotographic photoreceptor means that a force exceeding adhesion between the surface of the electrophotographic photoreceptor and the remaining toner or paper powder adhered is exerted on foreign matters such as the remaining toner or paper powder to remove the adherent matter from the surface of the electrophotographic photoreceptor. Accordingly, the lower the wettability of the surface of the electrophotographic photoreceptor becomes, the easier the cleaning becomes. The wettability, namely, the adhesion of the surface of the electrophotographic photoreceptor can be expressed using a surface free energy (which has the same meaning as a surface tension) as an index.
The surface free energy (γ) is a phenomenon which an intermolecular force, a force acting between molecules constituting a substance, causes on the outermost surface.
A toner that remains on the surface of the electrophotographic photoreceptor by adhesion or fusion without being transferred onto a transfer member is spread on the surface of the electrophotographic photoreceptor in the form of a film while steps from charging to cleaning are repeated. This phenomenon corresponds to “adhesion wettability” in the wettability. Further, a phenomenon in which a paper powder, a rosin, talc or the like is adhered to the surface of the photographic photoreceptor and the contact area with the electrophotographic photoreceptor is then increased to provide strong wettability also corresponds to “adhesion wettability”.
FIG. 17 is a side view showing a state of adhesion wettability. In the adhesion wettability shown in FIG. 17, the relation between the wettability and the surface free energy (γ) is represented by Young's formula (I).γ1=γ2·cos θ+γ12  (I)
wherein    γ1: surface free energy on a surface of material 1    γ2: surface free energy on a surface of material 2    γ12: interface free energy of materials 1 and 2    θ: contact angle of material 2 to material 1
In the formula (I), reduction in wettability of material 2 to material 1 which means that θ is increased for less wetting is attained by increasing the interface free energy Y12 related with a wetting work of the electrophotographic photoreceptor and the foreign matters and decreasing the surface free energies γ1 and γ2.
When adhesion of foreign matters, water vapor and the like to the surface of the electrophotographic photoreceptor is considered in the formula (I), material 1 corresponds to the electrophotographic photoreceptor and material 2 to foreign matters respectively. Accordingly, when the electrophotographic photoreceptor is actually cleaned, the wettability on the right side of the formula (I), namely, the adhered condition of the toner, paper powder and the like as foreign matters to the electrophotographic photoreceptor can be controlled by controlling the surface free energy γ1 of the electrophotographic photoreceptor.
In the related art that defines a surface condition of an electrophotographic photoreceptor, a contact angle with pure water is used (refer to, for example, Japanese Unexamined Patent Publication JP-A 60-22131 (1985)). However, in regard to wetting of a solid and a liquid, the contact angle θ can be measured as shown in FIG. 17, but in case of a solid and a solid such as an electrophotographic photoreceptor and a toner or a paper powder, the contact angle θ cannot be measured. Accordingly, the foregoing related art disclosed in JP-A 60-22131 can be applied to wettability between a surface of an electrophotographic photoreceptor and pure water, but a relation between wettability and cleanability of a solid such as a toner constituting a developer or a paper powder cannot be explained satisfactorily.
The wettability between solids can be represented by an interface free energy between solids. With respect to the interface free energy between solids, the Fowkes's theory stating a non-polar intermolecular force is considered to be further extended to a component formed by a polar or hydrogen-bonding intermolecular force (refer to Kitazaki T., Hata T., et al.; “Extension of Fowkes's Formula and Evaluation of Surface Tension of Polymeric Solid”, Nippon Secchaku Kyokaishi, Nippon Secchaku Kyokai, 1972, vol. 8, No. 3, pp. 131-141). According to this extended Fowkes's theory, the surface free energy of each material is found from 2 to 3 components. The surface free energy in the adhesion wettability corresponding to the adhesion of the toner or the paper powder to the surface of the electrophotographic photoreceptor can be found from 3 components.
The surface free energy between solid materials is described below. In the extended Fowkes's theory, an addition rule of the surface free energy represented by formula (II) is assumed to be established.γ=γd+γp+γh  (II)in which    γd: dipole component (polar wettability)    γp: dispersion component (non-polar wettability)    γh: hydrogen-bonding component (hydrogen-bonding wettability).
When the rule of addition of the formula (II) is applied to the Fowkes's theory, the interface free energy γ12 between substance 1 and substance 2, both of which are solids, is determined as in the following formula (III).γ12=γ1+γ2−{2√(γ1d·γ2d)+2√(γ1p·γ2p)+2√(γ1h·γ2h)}  (III)in which
γ1: surface free energy of material 1
γ2: surface free energy of material 2
γ1d, γ2d: dipole components of material 1 and material 2
γ1p, γ2p: dispersion components of material 1 and material 2
γ1h, γ2h: hydrogen-bonding components of material 1 and material 2
The surface free energies (γd, γp, γh) of the components in the solid materials to be measured as represented by the formula (II) can be calculated by using known reagents and measuring adhesion with the reagents. Accordingly, with respect to material 1 and material 2, it is possible that the surface free energies of the components are found and the interface free energy of material 1 and material 2 can be found from the surface free energies of the components using the formula (III).
On the basis of the concept of the solid-solid interface free energy found in this manner, another related art controls wettability of a surface of an electrophotographic photoreceptor and a toner or the like using a surface free energy of the electrophotographic photoreceptor as an index (refer to Japanese Unexamined Patent Publication JP-A 11-311875 (1999). JP-A 11-311875 discloses that a surface free energy is defined in the range of from 35 to 65 mN/m to improve cleanability of a surface of an electrophotographic photoreceptor and realize a long life thereof.
According to the present inventors' investigations, however, in the test of photography in which an image is actually formed on, for example, a recording paper using an electrophotographic photoreceptor having the surface free energy in the range disclosed in JP-A 11-311875, damage considered to occur by contact with foreign matters such as a paper powder and the like is confirmed on the surface of the electrophotographic photoreceptor. Further, it is also confirmed that owing to insufficient cleaning caused by this damage, black streaks occurred on images transferred on the recording paper. There is a tendency that the damage generated on the surface of the electrophotographic photoreceptor is increased with the increase in surface free energy.
In still another technique, an amount (Δγ) of change in surface free energy according to duration of an electrophotographic photoreceptor is defined. However, in consideration of the facts that the amount (Δγ) of change is not determined by defining initial characteristics, for example, the surface free energy, of the electrophotographic photoreceptor and the amount (Δγ) of change varies depending on conditions such as an environment in image formation and a material of a transfer member, the amount (Δγ) of change is problematic in that it might include an uncertain element and is therefore inappropriate as a designing standard in actual designing of an electrophotographic photoreceptor.
Further, in an organic photoreceptor, in order to control the surface free energy on the surface of the photoreceptor as in the technique disclosed in JP-A 11-311875, it is necessary to control the kind and the blending amount of a binder resin used for the photosensitive layer as a surface layer. However, this results in a problem that the sensitivity and the light responsiveness of the photoreceptor are lowered depending on the kind or the blending amount of the binder resin.
Since the sensitivity and the light responsiveness of the photoreceptor depends on the charge-transporting ability of the charge-transporting substance as described above, it is considered that lowering of the sensitivity and the light responsiveness can be suppressed by using a charge-transporting substance of high charge-transporting ability. However, the charge-transporting ability of the enamine compound as disclosed in JP-A 2-51162, JP-A 6-43674 or JP-A 10-69107 is insufficient and no sufficient sensitivity and light responsiveness can be obtained even by the use of the enamine compounds. Particularly, no sufficient light responsiveness can be maintained under a low temperature circumstance, and image having practically sufficient image density can not be formed. Further, as in the photoreceptor disclosed in JP-A 7-134430, it may be considered to incorporate a polysilane and an enamine compound having a specified structure. However, a photoreceptor using the polysilane is sensible to light exposure, and brings about another problem of lowering the various characteristics as the photoreceptor when exposed to light, for example, during maintenance.
That is, even the combination of the constitution of the photoreceptor disclosed in JP-A 11-311875 and the constitution of a photoreceptor disclosed in JP-A 2-51162, JP-A 6-43674, JP-A 10-69107 or JP-A 7-134430 can not attain a photoreceptor that has excellent durability having high sensitivity and light responsiveness, excellent circumstantial stability with less change of electric characteristics caused by fluctuation of the circumstance, as well as excellent cleaning property and is capable of providing images of high quality for a long period of time.